As I mention above, the paper it is tied to *is* a transactional document, not a cover. There's just not enough printing or handwriting to glean context as to the nature of the transaction.
You yourself stated "There were numerous perf separations which I stabilized with archival mending tape"; perhaps someone else also tried to do so with a piece of old document. I am not saying that it is NOT a revenue usage, only that there are legitimate reasons for thinking it might have been a postal use. A scan of the back might be interesting.
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